Based on their characteristics, proteins synthesized in cells can be categorized into the following groups of proteins: (1) those localized in various intracellular organelles, such as, the nucleus, mitochondria, and cytoplasm; (2) those functioning on the cell membrane, such as, receptors and channel molecules; and (3) those functioning after being secreted to the outside of cells, such as, growth factors and cytokines. Among others, secretory proteins specifically play biologically important roles in growth of cells, regulation of differentiation and apoptosis, inflammatory reactions, intercellular interaction, etc. Thus, the secretory proteins are the preferred target molecules of diagnostic and therapeutic drugs for various diseases.
Tissues at fetal stages, as compared to adult tissues wherein most of the cells are in a resting phase, express various genes and result in dynamic changes, such as, cell growth, regeneration, differentiation, and cell death. For example, fetal liver is an essential tissue for hemopoiesis and expresses various important genes participating in the maintenance of life. Thus, fetal liver is a preferred target to isolate genes playing important roles in living cells. Furthermore, molecules expressed at high levels in fetal liver are predicted to play important roles in living cells.